Short-term variability of physico-chemical parameters and the estimated transport of filterable nutrients and chlorophyll-a in the urbanised Coombabah Lake and Coombabah Creek system, southern Moreton Bay, Australia

Abstract

Extensive urban development has occurred within the Coombabah Creek catchment and surrounds over the last two decades, resulting in concerns about degrading water quality. Water quality was investigated three times during summer and three times during autumn/winter at two sites: within Coombabah Lake and at several km downstream within Coombabah Creek. Physico-chemical parameters, suspended solids, chlorophyll-a and filterable nutrients (reactive PO4 3-, NO2 -, NO3 - and NH3 -) were measured hourly for 13 hours in order to compare the water quality under different tidal conditions at each site. Bathymetric and hydrological data were also collected, which allowed short-term nutrient loads to be estimated. From these measurements, the net transport of filterable nutrients and chlorophyll-a into Coombabah Lake and the intratidal variability of Coombabah Creek and Lake waters were determined. Physico-chemical parameters, suspended solids and chlorophyll-a concentrations demonstrated characteristic cyclic variations with the influence of tidal and diurnal cycles apparent. Despite elevated nutrient concentrations, chlorophyll-a values indicated an oligotrophic-mesotrophic environment, with concentrations ranging between 0.11-3.53 μg L-1. Maximum concentrations occurred during ebb tides, coinciding with periods of increased solar radiation. Elevated total suspended solids were observed during increased current velocities during low tide periods and greater wind speeds within the lake. Filterable nutrient concentrations and transport estimations also demonstrated tidal influences, with increased concentrations observed during sampled high tide phases, indicating increased inputs of nutrients originating from sources other than the creek and lake. Observed reactive PO4 3-, NOx and NH3 - concentrations exceeded Broadwater sub-region values within the Queensland Water Quality Guidelines (2006).